Thousands of staff at Google are protesting against the company's work with the Pentagon's surveillance drones program.

Called Project Maven, the program uses Google technology to automate the analysis of objects in the enormous amount of images that are captured by the Department of Defense's unmanned aerial vehicles (UAVs).

More than 3,100 staff signatures have been attached to a letter addressed to chief executive Sundar Pichai which is circulating inside Google with a request that the company pull out of the project.

According to a copy of the letter published by The New York Times, the letter states: "We believe that Google should not be in the business of war.

"Therefore we ask that Project Maven be canceled, and that Google draft, publicize and enforce a clear policy stating that neither Google nor its contractors will ever build warfare technology."

There have been almost 30,000 coalition strikes against targets in Iraq and Syria since the US-led intervention in 2014, the intelligence behind many of which is developed by analysis of UAV surveillance footage.

The letter was circulated after Google confirmed that its open-source machine learning software TensorFlow was being used by the Pentagon for a pilot project, in response to a report by Gizmodo.

Google said it had "long worked with government agencies to provide technology solutions".

A spokesperson added: "The technology flags images for human review, and is for non-offensive uses only. Military use of machine learning naturally raises valid concerns.

"We're actively discussing this important topic internally and with others as we continue to develop policies and safeguards around the development and use of our machine learning technologies."

The open-source TensorFlow software is widely used in machine learning applications which attempt to allow computers to be taught how to identify objects in video footage.

The algorithm identifies cars and trees and is designed to assist the limited number of human analysts detecting threatening objects in an increasingly unmanageable amount of footage.

Google is one of many companies working on the project, the spokesperson said, adding that the algorithms were not designed or used for face detection.

The Department of Defense said that Project Maven was initially focusing on identifying 38 classes of objects which were important for its analysts to detect as the coalition continues to target Islamic State forces.

The project's head, Marine Corp Colonel Drew Cukor, told an audience of military and technology experts that the defense sector needed to embrace artificial intelligence technologies such as machine learning, according to Gizmodo.